In state-of-the-art IC technology, bond pads consist of a multilayer aluminum metallization, generally with one or more layers of e.g. titanium or titanium nitride. Bond pads are present for attaching solder, wire or other bonding elements, especially constructed from aluminum, gold or copper.
Bond pads are typically disposed above one or more layers or stacks of brittle and/or soft intermetal dielectric materials, such as silicon oxides and organic materials.
Bond pad cracks can occur during ultrasonic wire bonding or even through probing. Cracks can lead to reliability problems, especially when low-k spin-on dielectrics, such as hydrogen silsesquioxane (HSQ), are used. Such dielectrics, and especially HSQ, are more brittle than other oxides, such as tetraethoxysilane-based oxides. In HSQ layers cracks can therefore more easily propagate than in other oxides. Similar problems occur with aerogels, organic polyimides, parylenes and the like, which all have low dielectric constants as compared to silicon oxides, but are structurally and mechanically weaker than these oxides.
In the art, there is a need for structures or methods to prevent or at least reduce the occurrence of bond pad cracking.
It has been proposed in EP-A-0 875 934 to dispose a patterned reinforcing structure in a dielectric layer disposed under the bond pad. The basic principle laid down in this document is that through the use of metal grids mechanical reinforcement of the dielectric stack can be achieved and damage due to bonding can be prevented. More in detail, this known reinforced structure is manufactured by forming a metal layer, patterning the metal layer in a predetermined area in accordance with a predetermined pattern having a plurality of vacant areas, forming a dielectric layer above the patterned metal layer, and filling the vacant areas in the patterned metal layer. Finally, a bond pad is formed on the dielectric layer above the patterned metal layer.
The known reinforcing structure may be a joined or interconnected grid or a crosshatch structure with a plurality of voids or vacant areas for containing and accommodating a large portion of weak dielectric material such as the said HSQ and the like. The grid structure is planar with a thickness below the thickness of the intermetal dielectric stack. In another embodiment, the reinforcing structure includes a repeating and non-interconnected pattern such as a crucifix pattern arranged in a regular manner. Other structures such as spirals have been described as well.
The present inventors have intensively studied two of the structures known from EP-A-0 875 934:                the crosshatch structure, wherein at each metal level, under the bond pad, a crosshatch grid of metal was inserted to confine mechanically relatively weak HSQ into square reservoirs created by the grid; and        the crucifix structure. This structure provides a more open metal pattern as compared with the crosshatch structure, allowing HSQ to flow more easily into the voids.        
In the crosshatch structure, the metal line widths and spacings were designed to confine much of the HSQ into the reservoirs while minimizing the area of each reservoir, so that the HSQ layer is spared the direct mechanical impact of bonding. Vias were formed only at the pad periphery.
In the crucifix structure, the more open pattern allows the HSQ to flow more easily in the voids present in the structure. By virtue thereof, the amount of HSQ remaining over the metal lines is reduced slightly further as compared to the crosshatch structure.
Both known structures have, however, a continuous TEOS dielectric layer between the metal layers, which dielectric layer can be cracked due to bonding or probing.